1. Field of the Invention
This invention relates to a method and apparatus for separating a mixture by adsorption. The term "adsorption" used herein is intended to mean a phenomenon of retaining the components in a fluid on a solid no matter how the retaining mechanism may be such as an adsorption, absorption, sorption, combination, ion exchange or the like. More particularly, the present invention relates to a method and apparatus for separating, by column chromatography, a solution containing two or more components into at least two fractions, i.e., a fraction which is rich with a component susceptible to adsorption by an adsorbent and a fraction enriched with a component which is hard to be adsorbed by the same adsorbent.
2. Description of the Prior Art
The separation of mixtures by adsorption using various types of adsorbents has been known and widely applied industrially. The simplest adsorption separator is a single column packed with adsorbents. In order to conduct the separation using a column, there is first fed from the top of the column a starting solution which contains both a component which is susceptible to adsorption by adsorbents and a component which is difficult to adsorb, followed by feeding a desorbent. Typical of the procedure using such column, there is first discharged from the bottom a solution enriched with the hard-to-adsorb component, then a solution which is rich with both the hard-to-adsorb component and the easy-to-adsorb component, and finally a solution which is rich with easy-to-adsorb component. The solution containing both components which is discharged at the intermediate stage of the procedure is recycled to the top of the column after being mixed with the starting solution. This separation procedure is capable of being conducted by use of simple apparatus and is simple in operation but it is not good at the efficiency of separation. In addition, the procedure generally requires large quantities of desorbent, so that the concentration of the hard-to-adsorb component or the easy-to-adsorb component in a solution obtained as a product becomes diluted. In U.S. Pat. No. 3,416,961, there is described an improved method in which the solution discharged from the bottom of a column is divided into a number of fractions. The fractions other than those useful as products are temporarily stored in tanks for a subsequent feed from the tanks to the column in the intervals of feeding the starting solution and the desorbent to the column.
In U.S. Pat. No. 4,022,637, there is described a method wherein a solution discharged from the bottom of a column is divided into five fractions, of which two fractions are used as a product and the remaining three are recycled to the column. Two fractions of the three are directly recycled to the top of the column and the other one is stored in a tank and fed to the top of the column subsequent to the starting solution.
In these methods described hereinabove, all the solutions in the column flow down from the bottom of the column. Further, an additional tank or tanks are required for the fraction or fractions other than the product, which have to be stored temporarily therein.
Japanese Laid-open Patent Application No. 53-149870, describes a method in which fractions other than a product are directly recycled to the top of a column without use of any storage tank. In this method, however, the fractions as product are withdrawn only from the bottom of the column. A modification is also described in which two columns are combined and fractions useful as product are withdrawn from the bottom of the respective columns. In either method, two fractions are recycled every cycle of the operation.